A fingerprint identification device is used in an electronic product to read a fingerprint image. After the fingerprint identification device confirms that the read fingerprint image complies with a predetermined fingerprint file, the electronic product is unlocked. In the early stage, the fingerprint identification device is used in an access control system. Recently, with increasing development of the smart phone technologies, smart phones are usually equipped with fingerprint identification devices. In comparison with the fingerprint identification device of the access control system, the thickness of the fingerprint identification device is gradually reduced because the trend of designing the smart phone is toward slimness. Therefore, it is important to increase the accuracy of the fingerprint identification device to sense the fingerprint image.
FIG. 1 is a schematic view illustrating the structure of a conventional fingerprint identification module. As shown in FIG. 1, the conventional fingerprint identification module 10 comprises a fingerprint sensor die 11, a rigid circuit board 12, a flexible circuit board 13, a mold compound layer 14, a cover plate 16 and a reinforcement plate 17. The reinforcement plate 16 is used for increasing the flatness of the flexible circuit board 13. The fingerprint sensor die 11 is attached on the rigid circuit board 12 through an adhesive layer 12A. Moreover, the fingerprint sensor die 11 is connected with the rigid circuit board 12 through wires W. The rigid circuit board 12 and the flexible circuit board 13 are combined together through a soldering layer 13A. The flexible circuit board 13 and the reinforcement plate 17 are combined together through an adhesive layer 17A. The cover plate 16 is attached on the mold compound layer 14 through an adhesive layer 15.
For example, the fingerprint sensor die 11 is a capacitive fingerprint sensor die. When a user's finger is placed on the cover plate 16, the fingerprint sensor die 11 senses the fingerprint image and transmits the fingerprint image to an electronic device through the rigid circuit board 12 and the flexible circuit board 13. Consequently, the fingerprint image can be recognized by the electronic device. However, for those skilled in the art, the conventional fingerprint identification module 10 of FIG. 1 still has some drawbacks. Generally, in case that distance between a sensing surface of the fingerprint sensor die 11 and the finger is smaller, the accuracy of the fingerprint sensor die 11 (e.g., the capacitive fingerprint sensor die) for sensing the fingerprint is higher. As shown in FIG. 1, the overall thickness of the adhesive layer 15 and the cover plate 16 is D1, and the thickness of the mold compound layer 14 overlying the fingerprint sensor die 11 is D2. The distance between the finger and the sensing surface of the fingerprint sensor die 11 is equal to the sum of D1 and D2. For manufacturing the conventional fingerprint identification module 10, a molding process is firstly performed to form the mold compound layer 14 to encapsulate the fingerprint sensor die 11 and the wires W, and then the adhesive layer 15 is coated on the top surface of the mold compound layer 14 to attach the cover plate 16. During the process of forming the mold compound layer 14, the thickness D2 of the mold compound layer 14 overlying the fingerprint sensor die 11 is unavoidable. The thickness D2 increases the distance between a sensing surface of the fingerprint sensor die 11 and the finger.
The uneven thickness of the mold compound layer 14 is another drawback of the fingerprint identification module 10 of FIG. 1. Since it is difficult to precisely control the stress of the package structure during the process of forming the mold compound layer 14, the thickness of the mold compound layer 14 is not uniformly distributed. Consequently, the surface of the mold compound layer 14 is suffered from warpage. When the warpage of the mold compound layer 14 occurs, the thickness of the adhesive layer 15 overlying the mold compound layer 14 is also uneven. That is, the two lateral regions are thicker than the middle region, or the two lateral regions are thinner than the middle region. Due to the uneven thickness, the fiducial capacitance sensed by the fingerprint sensor die 11 is inconsistent. Under this circumstance, the possible of resulting in misjudgment is increased.